"Windows Vista introduces a new concept in adding memory to a system. Windows ReadyBoost lets users use a removable flash memory device, such as a USB thumb drive, to improve system performance without opening the box. Windows ReadyBoost can improve system performance because it can retrieve data kept on the flash memory more quickly than it can retrieve data kept on the hard disk, decreasing the time you need to wait for your PC to respond. Combined with SuperFetch technology, this can help drive impressive improvements in system responsiveness."

DarkPenguin

The 300mb/s is the interface speed. Not the drive speed. You can't get data off the drive at that speed. So even if you ignore the time it takes to seek to the data you probably can't move it off the disk all that much faster.